Some pyrotechnic devices use a percussion primer to begin the device's ballistic event. Typically, the percussion primer is actuated by a high pressure gas or by a mechanical firing mechanism. In terms of mechanical firing mechanisms, the hook-type sear design is frequently used because of its simplicity and minimal space requirements. A conventional hook-type sear mechanical firing mechanism is shown in its cocked position if FIG. 1 and in its release position in FIG. 2. In both figures, the firing mechanism is referenced generally by numeral 10.
Firing mechanism 10 includes an annular housing 12 that is centrally bored to define two diameters at 13A and 13B to support a firing pin 14 and initiator pin 16 that share a common longitudinal axis with that of the housing's bore 13. A spring 18 is disposed between an outer annular shoulder 12A of housing 12 and an annular flange 14A of firing pin 14. In the cocked position (FIG. 1), spring 18 is slightly compressed to thereby apply opposing forces FC to flanges 12A and 14A. In this position, spring 18 is held in place as a flange 16A (formed on initiator pin 16) that is larger in diameter than smaller bore 13A abuts housing 12 as shown. Firing pin 14 and initiator pin 16 are locked together by the combination of (i) complementary hooks 14B and 16B, respectively, and (ii) the narrowly-bored region 13A of housing 12 that constrains firing pin 14 and initiator pin 16 from radial movement. In operation, initiator pin 16 is pulled longitudinally in the direction indicated by arrow 20 in FIG. 2. During this process, spring 18 undergoes further compression. Once hook 16B clears narrowly bored region 13A and enters the larger bored region 13B, initiator pin 16 can disengage from firing pin 14. Once this occurs, the force of spring 18 is released and firing pin 14 moves in the direction of arrow 22 to start a ballistic event.
One problem with the hook-type sear design is the fabrication cost associated with the manufacture of hooks that will perform reliably, e.g., not break, not jam together when they are supposed to separate, etc. Conversely, if short cuts are take in the manufacturing process to save money, the result is an unreliable coupling/release assembly. Further, the hook-type design has an inherent weakness since material must be notched out to create the hook.